Crossing the Boundary: The Role of Evergreen Forest Vegetation on the Bowen Ratio in the Southeastern United States


Shawn Serbin1*, Jeremiah Anderson1, Ken Davidson1,2, Alistair Rogers1


1Environmental and Climate Sciences Department, Brookhaven National Laboratory, Upton, NY; 2Department of Ecology and Evolution, The State University of New York–Stony Brook, NY


Vegetation and atmosphere processes are coupled through complex physical, chemical, and biological interactions. These processes regulate surface–atmosphere exchanges of carbon, water, and energy, which exert strong controls over emergent weather and climate patterns and are subject to land–surface feedbacks operating across a wide range of spatial and temporal scales. This surface–atmosphere coupling is primarily governed by sensible (conductive) and latent (evaporation and transpiration) heat fluxes and associated carbon cycle processes, which are primarily regulated by plant photosynthesis and transpiration. While it is known that there are important biotic and abiotic controls on these processes, the representation of vegetation structure, function, and water and energy cycling in land–surface and boundary layer modeling is typically incomplete, which results in large model uncertainties that limit understanding of the key connections between the land and atmosphere. In this project, researchers set out to address a key driver of model uncertainty by studying biotic and abiotic controls on the Bowen ratio—the ratio between sensible and latent heat flux that describes the amount of energy transfer between the surface and atmosphere. To connect measurements to the larger forest ecosystem-scale fluxes within the southeastern United States, the team conducted research at the National Ecological Observatory Network (NEON) Talledega National Forest eddy covariance site, an evergreen forest dominated by longleaf (Pinus palustris) and loblolly (Pinus taeda) pines, with mixed oak species in the understory. Researchers developed and instrumented 30 evergreen trees with a prototype sap-flow sensor system along an existing instrumented soil moisture gradient. The sap-flow sensors provide continuous measurements of tree water cycling within the footprint of the flux tower, allowing connection of seasonal variation in ecosystem-scale carbon, water, and energy fluxes to tree-scale transpiration. Using prototype sensors, the team identified important differences in tree sap flux across seasonal timescales, between species, and across a moisture gradient that are strong drivers of the temporal and spatial variation in the Bowen ratio.